Due to a depletion of fossil fuels, there are considerable studies to find alternative energy sources such as solar energy, biomass, and organic wastes among others. These renewable energy sources can be stored into molecular hydrogen or electricity as a clean energy carrier through wastewater electrolysis cell or wastewater fuel cell in the water-energy nexus framework. To this end, urea oxidation reaction (UOR) has been interrogated to enhance the energy efficiency of electrochemical hydrogen production or to directly harvest electricity in urea fuel cell. These approaches are advantageous to recover chemical energy from wastes during urea-rich wastewater treatment. Nickel-based catalysts are up-to-date the most widely investigated materials as the UOR catalyst. This presentation introduces our recent approaches for preparation of the UOR electrocatalysts by i) galvanostatic electro-deposition of mixed metal (oxy)hydroxide (NiFeOx or NiCoOx) with variable mixing ratios, ii) potentiostatic dealloying of commercial alloys (e.g. Ni-Fe, Fe-Ni, and Ni-Cr-Co) under variable pH, and iii) one-pot thermochemical treatment of NiFe alloy foam (NFF), both as substrate and catalyst source to create a self-supporting UOR electrocatalyst. The secondary elements (Fe and Co) were found to accelerate the formation of NiOOH (the active site for UOR), by partial charge transfer among the mixed metals. Electro-dissolution in neutral pH could also invigorated the UOR activity of Ni-Fe alloy. In particular, the mixed Co elements effectively reduced the UOR overpotential, to benefit the power density in the urea fuel cell. On the other hand, oxalic acid treatment of NFF generated prismatic NiFe-oxalate (O-NFF) which marked surprisingly small overpotential and Tafel slope for UOR. The coexistence of Fe and oxalate ligand simultaneously shift the surface electronic structure which in-turn affected the adsorption of reactants and desorption of products. The UOR on O-NFF could be utilized instead of oxygen evolution reaction to enhance the energy efficiency of hydrogen production.

Direct urea oxidation reaction on Ni-based catalysts for environmental and energy applications

ACS Spring 2022

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